Current trends suggest solar energy will play an important role in future energy production. Silicon has been and remains the traditional solar cell material of choice. While silicon is a highly abundant material, it requires an energy intensive process to purify and crystallize. Furthermore, installations of silicon cells require heavy glass protection plates, which reduce residential applications. Recently, commercial interest is beginning to shift towards thin film cells. Material, manufacturing time, and weight savings are driving the increase in thin films. CIGS solar cells have the highest efficiency among thin film technologies.
CIGS thin film solar cells have achieved an efficiency of over 20% in the laboratory. Several companies have attempted to scale-up and commercialize the technology, but none has yet made a significant impact. The difficulty has been developing a viable manufacturing process. There are two leading technologies that are being pursued: co-deposition which has produced the record lab cell efficiencies, and the two-step process of depositing a metal precursor containing the entire thickness of metals and then selenizing it. There are several variations in the latter process including use of H2Se gas or evaporated Se as the selenizing agent, and deposition of the precursor by physical vapor deposition, electroplating or from an “ink” bearing liquid. While many factors come into play, a simplistic overview is that the co-deposition process is difficult to control in large areas, and the selenization processes are slow. A particular difficulty with the co-deposition process is that the established technology for coating large areas in a manufacturing environment is sputtering. While the metals can be sputtered, sputtering Se at the required rates is a challenge. Efforts to sputter the metals in the presence of a Se vapor provided by an evaporation source have resulted in poisoning of the metal targets. A combination of sputtering for the metals and evaporation for Se may overcome this difficulty. However, sputtering and evaporation are most effective in different pressure regimes, which make them difficult to use in the manufacturing process. Furthermore, Se is known to permeate the deposition environment and interfere with the sputtering tool.
The best co-deposited films are made with either a 2-step or 3-step process. In the 2-step process, a first layer is deposited which is basically a Cu-rich layer of Cu, Ga and Se, and could contain a small amount of In. The role of this layer is to form an initial large grain structure to propagate the growth of large grains for the finished CuInGaSe2 film. The film is grown at a substrate temperature of about 300° C. which fosters the growth of binary phases. Under typical conditions the predominant phases are Ga2Se3 and Cu3Se2. For equal numbers of the binaries Ga2Se3 and Cu3Se2, the Cu/Ga ratio is 1.5, and the Se content is 50%. Other Cu phases can form depending on the Se flux. CuSe and Cu2Se can also form. While Ga2Se3 remains the dominant phase for Ga under most conditions, Ga2Se can form under low Se flux as well. This compound is volatile and results in loss of Ga from the film. To prevent this, high Se fluxes are typically used, but this is wasteful of Se. One then must attempt to tune the conditions to produce the desirable film while maximizing the tradeoff in Ga and Se waste. In the 3-step process the first step is deposition of a Group III-VI compound, usually GaxSey. Cu is not used in the first step because the III-VI selenides form a smooth and dense surface which improves overall film uniformity and yield. In the second step the Cu is added to accomplish the Cu-rich larger grain growth, and the proper quantities of all three metals are added in the third step to result in desired metal ratios for the finished film. These steps also provide means of grading the Ga composition.
Deposition technologies that can match the performance of small area CIGS cells in large areas and production volumes are needed. The best small area cells are made by co-evaporation of the constituent elements. However, it has been difficult to use this approach in commercial production because there are no suitable large area production viable evaporation tools as discussed above.
In summary, the highest efficiency achieved for CIGS solar cells involves a multi-step co-deposition process that is difficult to scale up to a viable manufacturing process. Alternative deposition approaches that are easier to manufacture do not attain the performance level of the multi-step co-deposition process. Consequently, the commercialization of CIGS technology lags that of other thin film technologies despite its higher demonstrated efficiency in the laboratory.